Using a screen designed to identify yeast mutants specifically defective in recombination between homologous chromosomes during meiosis, we have obtained new alleles of the meiosis-specific genes, HOP1, RED1, and MEK1. In addition, the screen identified a novel gene designated MSH5 (MutS homolog 51. Although Msh5p exhibits strong homology to the MutS family of proteins, it is not involved in DNA mismatch repair. Diploids lacking the MSH5 gene display decreased levels of spore viability, increased levels of meiosis I chromosome nondisjunction, and decreased levels of reciprocal exchange between, but not within, homologs. Gene conversion is not reduced. Msh5 mutants are phenotypicaUy similar to mutants in the meiosis-specific gene MSH4 (Ross-Macdonald and Roeder 1994}. Double mutant analysis using rash4 rash5 diploids demonstrates that the two genes are in the same epistasis group and therefore are likely to function in a similar process--namely, the facilitation of interhomolog crossovers during meiosis.
Immunoprobes that combine a fluorescent label with a 1.4-nm gold cluster compound have been prepared by covalent conjugation with polyclonal antibody Fab' fragments. These new immunoconjugates allow the collection of two complementary sets of data, from fluorescence and electron microscopy, from a single labeling experiment. We find that incorporation of one or more fluorescein moieties into the coordinated ligands of the 1.4-nm Nanogold gold cluster label yields a stable, dual-function immunolabel in which fluorescence quenching is negligible. In a second synthetic strategy, Nanogold and fluorescein were separately covalently conjugated to Fab' fragments to yield a probe with very similar properties. With the use of Fab' fragments, the entire probe is smaller than a whole IgG molecule, and it exhibited excellent penetration properties. It was used to localize the pre-mRNA splicing factor SC35 in the HeLa cell nucleus by both fluorescence and electron microscopy.
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